Nanostructured metallic cores with extremely low loss and controlled permeability

Magnetic anisotropy perpendicular to the ribbon axis was induced in nanocrystalline FeCuNbSiB ribbons by crystallization under tensile stress, and the relative permeability of the ribbon was approximately 250. Toroidal cores for a choke coil were prepared from the ribbons. The magnetic loss decreased with increasing the core diameter and it was found that deterioration of magnetic properties due to formation of a toroidal core was suppressed by making the core diameter larger than the critical diameter determined from the values of magnetostriction and induced anisotropy. As a result, toroidal cores with extremely low loss were achieved. The permeability of the cores developed was kept constant up to 1 MHz and their magnetic loss was much smaller than the loss values reported previously for nanostructured and amorphous cores with low permeability. The measured magnetic loss agreed with the calculated classical eddy current loss. In addition, the permeability and magnetic toss were kept constant up to the dc bias field, being three times higher than that applicable to a ferrite cut core with the same permeability value.